Hydraulic circuits used for outrigger cylinder drive in the related art include, for instance, the hydraulic circuit disclosed in Japanese Laid Open Utility Model Publication No. S63-4772.
In conjunction with the hydraulic circuit disclosed in this publication, the bottom chambers or the rod chambers of outrigger cylinders disposed to the front, the rear, the left side and the right side of the vehicle are individually made to communicate via hydraulic pilot switching valves. In response to a switching operation at the switching valve, pressure oil is allowed to flow to a desired hydraulic cylinder while cutting off the flow of pressure oil to the other hydraulic cylinders. This system makes it possible to operate the outriggers on the front side, the rear side, the left side and the right side independently of one another.
However, if high pressure oil is applied to hydraulic cylinders in the circuit disclosed in the publication described above, in which the oil flow is cut off with the switching valve, the oil may leak from the switching valve and in such a case, it may not be possible to hold the vehicle body in a jacked up state. While a leakless switching valve may be utilized to avoid this problem, the use of the leakless switching valve is bound to be costly.